Ion guide and mass spectrometer

ABSTRACT

A curved ion guide ( 2 ) includes four curved rod electrodes ( 201 - 204 ) arranged around a curved central axis (O), two deflecting auxiliary electrodes ( 205, 206 ) which are located on a plane P on which the curved central axis (O) lies and which face each other across the axis (O), and two focusing auxiliary electrodes ( 207, 208 ) which are located on a curved surface orthogonal to the plane P and including the axis (O) and which face each other across the axis (O). Ions are focused by the effect of an electric field created by radio-frequency voltages applied to the curved rod electrodes, and a deflecting electric field having the effect of curving ions along the axis (O) is created by direct-current voltages applied to the deflecting auxiliary electrodes. Furthermore, a focusing direct-current electric field having the effect of pushing ions from the vicinity of the focusing auxiliary electrodes toward the axis (O) is created by a direct-current voltage having the same polarity as that of the ions and applied to the focusing auxiliary electrodes. The spatial spread of ions having large amounts of energy is suppressed by the effect of this focusing direct-current electric field, and the ions efficiently arrive at the exit end of the ion guide.

TECHNICAL FIELD

The present invention relates to an ion guide for transporting ionswhile focusing them, as well as a mass spectrometer using the ion guide.

BACKGROUND ART

In a mass spectrometer, an ion optical element called the “ion guide” isused for focusing ions coming from the previous stage, accelerating themin some cases, and sending them into a mass analyzer, such as aquadrupole mass filter. An ion guide generally has a multi-polestructure with four or eight cylindrical (or tubular) rod electrodesarranged parallel to each other around an ion beam axis. Normally, inthe quadrupole or octapole ion guide, the same radio-frequency (RF)voltage is applied to one pair of rod electrodes facing each otheracross the ion beam axis, while another RF voltage, which is identicalin amplitude and opposite in phase to the aforementioned RF voltage, isapplied to another pair of rod electrodes neighboring the aforementionedpair in the circumferential direction. The thus applied RF voltagescreate an RF electric field in the space surrounded by the rodelectrodes, and the ions are transported to the subsequent stage whilebeing oscillated in this RF electric field.

In an ion guide disclosed in Patent Document 1, virtual rod electrodes,each of which consists of a plurality of plate electrodes arrayed alongthe ion beam axis, are used in place of the rod electrodes. In thevirtual-rod configuration, a direct-current (DC) electric field having apotential gradient along the ion beam axis can be created so as toaccelerate, or conversely, decelerate ions while exploiting theadvantage of high ion-focusing performance of the multipole ion guide.

As already explained, ion guides are primarily used to transport variousions produced by an ion source to a mass analyzer. However, theparticles introduced into the ion guide normally contain not only ionsoriginating from a sample, but also neutral particles, such as thesample molecules which have not been ionized in the ion source. Suchneutral particles, if allowed to reach the mass analyzer, will cause ameasurement noise. Furthermore, they will also contaminate the massanalyzer. Given these problems, a curved ion guide using curved rodelectrodes has been conventionally used to remove neutral particles inthe course of their travel through the ion guide (for example, refer toPatent Document 2 or 3).

FIG. 8 is a schematic perspective view of one example of the curved ionguide. As shown, this ion guide 2 has four curved rod electrodes 201,202, 203 and 204. Due to the effect of the RF electric field, ions whichhave originated from a sample follow a curved path along the shape ofthe ion guide, whereas neutral particles, which have no electric chargesand will not be affected by the RF electric field, travel straightlythrough the ion guide 2, to be eventually eliminated by being dischargedfrom the ion guide 2 or coming in contact with the curved rod electrodes201-204.

Since the ions introduced into the ion guide 2 have certain amounts ofenergy, it is actually difficult to achieve both the focusing andcurving of the ions along the curved path by using only the RF electricfield. To address this problem, a curved ion guide disclosed in PatentDocument 3 not only employs the curved shape of the rod electrodes butalso applies a deflecting DC voltage to the curved rod electrodes orauxiliary electrodes provided independently of the curved rodelectrodes, so as to create, in the space surrounded by the curved rodelectrodes, a DC electric field which acts on the ions and curves themtoward the inside of the curved path (as indicated by the arrow R inFIG. 8).

FIGS. 9 and 10 are configuration diagrams of the curved rod electrodesand the auxiliary electrodes described in Patent Document 3 as well asthe circuit blocks for applying voltages to those electrodes. The systemshown in FIG. 9 has no auxiliary electrodes. The thick white arrow inthis figure points toward the inside of the curved path in the curvedion guide 2 (i.e. inward along the radial direction of the curvedcentral axis, which is a segment of an arc). The voltage sources 501-504apply an RF voltage V_(RF) to the two curved rod electrodes 202 and 204facing each other among the four curved rod electrodes 201-204, as wellas an RF voltage −V_(RF) with the same amplitude and opposite polarityto the other two curved rod electrodes 201 and 203. As a result, an RFelectric field for focusing ions while oscillating them in thepreviously described manner is created in the space surrounded by thecurved rod electrodes 201-204. The voltage sources 501-504 also apply aDC voltage −V_(DEF) whose polarity is opposite to that of an ion to beanalyzed (which is a positive ion in the present example) to the twocurved rod electrodes 201 and 202 located on the inside of the curvedpath, as well as a DC voltage V_(DEF) having the same polarity as thatof the ion to be analyzed to the two curved rod electrodes 203 and 204located on the outside of the curved path. As a result, a DC electricfield for attracting ions toward the inside of the curved path, i.e. inthe direction indicated by the thick white arrow in the figure, iscreated in the space surrounded by the curved rod electrodes 201-204.

The system shown in FIG. 10 has auxiliary electrodes 205 and 206. Thevoltage source 511 and 512 apply an RF voltage V_(RF) to the two curvedrod electrodes 202 and 204 facing each other among the four curved rodelectrodes 201-204, as well as an RF voltage −V_(RF) with the sameamplitude and opposite polarity to the other two curved rod electrodes201 and 203. The voltage source 514 applies a DC voltage −V_(DEF) whosepolarity is opposite to that of an ion to be analyzed to the auxiliaryelectrode 205 located on the inside of the curved path. The voltagesource 513 applies a DC voltage V_(DEF) having the same polarity as thatof the ion to be analyzed to the auxiliary electrode 206 located on theoutside of the curved path. As a result, similar to the system of FIG.9, a DC electric field for attracting ions toward the inside of thecurved path is created, in the form of being superposed on theion-focusing RF electric field, in the space surrounded by the curvedrod electrodes 201-204.

By applying appropriate deflecting DC voltages to either the curved rodelectrodes or the auxiliary electrodes in the previously describedmanner, it is possible to curve ions along the curved path of the ionguide 2 and guide them to the exit end so as to improve the iontransmission efficiency. However, such conventional systems have thefollowing problem.

That is to say, the DC electric field which acts on the ions in theradial direction within the inner space of the ion guide 2 in thepreviously described manner functions as an energy filter which allowsthe passage of ions only within a specific range of kinetic energy.Accordingly, the transmission efficiency of the ions deteriorates if thevariation in the kinetic energy the ions introduced into the ion guide 2is relatively large. To avoid this situation, it is necessary to reducethe relative variation of energy by comparatively increasing the kineticenergy of the ions introduced into the ion guide 2. For the ion guidedisclosed in Patent Document 3, a difference in the ion transmissionefficiency depending on the presence or absence of the deflecting DCelectric field has been investigated for an ion having a considerablyhigh kinetic energy of 100 eV. However, a study by the present inventorhas revealed that, when ions with such a high kinetic energy areintroduced into a curved ion guide, it is difficult to adequately focusthe ions by using only the RF electric field. This constitutes a causeof deterioration in the ion transmission efficiency.

BACKGROUND ART DOCUMENT Patent Document

-   Patent Document 1: JP-A 2000-149865-   Patent Document 2: JP-B 3542918-   Patent Document 3: US-A1 2009/0294663

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The present invention has been developed to solve the previouslydescribed problem, and its objective is to provide a curved ion guidewhich exhibits a high ion-focusing performance and thereby achieves ahigh level of ion transmission efficiency even if the amount of kineticenergy of the introduced ions is large. An objective of the massspectrometer according to the present invention is to enhance thedetection sensitivity by using a curved ion guide with improved iontransmission efficiency.

Means for Solving the Problems

The first aspect of the present invention aimed at solving theaforementioned problem is an ion guide for transporting ions along acurved path while focusing the ions, including:

a) 2n pieces of curved rod electrodes (n is an integer equal to orgreater than two) arranged around a curved central axis; and b) avoltage generator for applying voltages to the 2n pieces of curved rodelectrodes as follows: radio-frequency voltages with opposite polaritiesare applied to any two curved rod electrodes neighboring each other inthe circumferential direction among the 2n pieces of curved rodelectrodes; a deflecting direct-current voltage is applied to at leastone of the curved rod electrodes in addition to the radio-frequencyvoltages, so as to attract ions in the space surrounded by the 2n piecesof curved rod electrodes toward the inside of the curvature of thecurved central axis in a plane orthogonal to the curved central axis;and a focusing direct-current voltage is applied to at least two curvedrod electrodes facing each other across the curved central axis,exclusive of the curved rod electrodes to which the deflectingdirect-current voltage is applied, in addition to the radio-frequencyvoltages, so as to push the ions in the space surrounded by the 2npieces of curved rod electrodes toward the curved central axis fromoutside, in the plane orthogonal to the curved central axis and along aline orthogonal or oblique to the direction in which the ions areattracted due to the deflecting direct-current voltage.

In the first aspect of the present invention, n is an integer equal toor greater than two, and in principle, it has no upper limit. However,in practice, n should preferably be within a range from two to four;i.e. the curved rod electrodes should preferably be constructed as aquadrupole, hexapole or octapole structure.

In one mode of the ion guide according to the first aspect of thepresent invention, the ion guide has a quadrupole structure of n=2 withfour curved rod electrodes arranged in such a manner that one pair ofthe curved rod electrodes facing each other across the curved centralaxis have the centers thereof located on a flat plane on which thecurved central axis lies while the other pair of the curved rodelectrodes have the centers thereof located on a curved surfaceorthogonal to the flat plane and including the curved central axis, andthe voltage generator applies the deflecting direct-current voltage toone or both of the pair of the curved rod electrodes having the centerthereof located on the flat plane and the focusing direct-currentvoltage having the same polarity as that of an ion to be analyzed to theother pair of the curved rod electrodes.

In the ion guide according to the first aspect of the present invention,ions introduced into the space surrounded by the 2n pieces of curved rodelectrodes experience not only the focusing effect due to theradio-frequency electric field, but also a force due to thedirect-current electric field created by the curved rod electrode towhich the focusing direct-current voltage is applied, and this forcecompresses the ions into a region near the curved central axis in adirection orthogonal or oblique to the radial direction in which theions are gradually curved. Therefore, even in the case where ions whichhave been introduced with considerably large amounts of kinetic energytravel along a curved path under the effect of the deflectingdirect-current electric field, the ions are prevented from spreading, sothat they can reach the exit end of the ion guide with high efficiency.Thus, a high level of ion transmission efficiency can be achieved.

In another mode of the ion guide according to the first aspect of thepresent invention, each of the curved rod electrodes is a curved virtualrod electrode composed of an array of plate electrodes arranged alongthe curved central axis, and the voltage generator applies, as thefocusing direct-current voltage, a voltage having the same polarity asthat of an ion to be analyzed and a voltage having an opposite polarity,to the array of the plate electrodes constituting one curved virtual rodelectrode so that these two voltages alternate in the array.

In this configuration, the direct-current electric field created by thefocusing direct-current voltage has the effect of focusing the ions atevery other plate electrode of the curved virtual rod electrode whenions are travelling along the curved path. This system functions as aplurality of serially arranged ion lenses, whereby the ions can beefficiently transported.

The second aspect of the present invention aimed at solving theaforementioned problem is an ion guide for transporting ions along acurved path while focusing the ions, including:

a) 2n pieces of curved rod electrodes (n is an integer equal to orgreater than two) arranged around a curved central axis, with none ofthe curved rod electrodes being located on a flat plane on which thecurved central axis lies;

b) a deflecting auxiliary electrode having a curved shape, located onthe flat plane on which the curved central axis lies and between thecurved rod electrodes neighboring each other in the circumferentialdirection;

c) a focusing auxiliary electrode having a curved shape, located on acurved surface which is orthogonal or oblique to the flat plane andwhich includes the curved central axis and between the curved rodelectrodes neighboring each other in the circumferential direction;

d) a main voltage generator for applying radio-frequency voltages withopposite polarities to any two curved rod electrodes neighboring eachother in the circumferential direction among the 2n pieces of curved rodelectrodes; and

e) an auxiliary voltage generator for applying a deflectingdirect-current voltage to the deflecting auxiliary electrode so as toattract ions in the space surrounded by the 2n pieces of curved rodelectrodes toward the inside of the curvature of the curved central axisin a plane orthogonal to the curved central axis, and for applying afocusing direct-current voltage to the focusing auxiliary electrode soas to push the ions in the space surrounded by the 2n pieces of curvedrod electrodes toward the curved central axis from outside, in the planeorthogonal to the curved central axis and along a line orthogonal oroblique to the direction in which the ions are attracted due to thedeflecting direct-current voltage.

Similar to the first aspect of the present invention, in the secondaspect of the present invention, n is an integer equal to or greaterthan two, and in principle, it has no upper limit. However, in practice,n should preferably be within a range from two to four; i.e. the curvedrod electrodes should preferably be constructed as a quadrupole,hexapole or octapole structure.

In one mode of the ion guide according to the second aspect of thepresent invention, the ion guide has a quadrupole structure of n=2 withone pair of the deflecting auxiliary electrodes facing each other acrossthe curved central axis and one pair of the focusing auxiliaryelectrodes facing each other across the curved central axis on a curvedsurface orthogonal to the flat surface, and the auxiliary voltagegenerator applies a deflecting direct-current voltage whose polarity isopposite to that of an ion to be analyzed to one of the deflectingauxiliary electrodes located on the inside of the curvature, adeflecting direct-current voltage having the same polarity as that ofthe ion to be analyzed to the other one of the deflecting auxiliaryelectrodes located on the outside of the curvature, and a focusingdirect-current voltage having the same polarity as that of the ion to beanalyzed to both of the focusing auxiliary electrodes.

In the ion guide according to the second aspect of the presentinvention, ions introduced into the space surrounded by the 2n pieces ofcurved rod electrodes experience not only the focusing effect due to theradio-frequency electric field, but also a force due to thedirect-current electric field created by the focusing auxiliaryelectrodes to which the focusing direct-current voltage is applied, andthis force compresses the ions into a region near the curved centralaxis in a direction orthogonal or oblique to the radial direction inwhich the ions are gradually curved. Therefore, even in the case whereions which have been introduced with considerably large amounts ofkinetic energy travel along the curved path under the effect of thedeflecting direct-current electric field, the ions are prevented fromspreading, so that they can reach the exit end of the ion guide withhigh efficiency. Thus, a high level of ion transmission efficiency canbe achieved.

In addition to the focusing direct-current voltage, a radio-frequencyvoltage for strengthening the effect of the radio-frequency electricfield may also be applied to the focusing auxiliary electrode.

A mass spectrometer according to the third aspect of the presentinvention aimed at solving the aforementioned problem is characterizedin that an ion guide according to the first or second aspect of thepresent invention is provided between an ion source and a mass analyzer.

By this system, ions produced by the ion source can be efficientlytransported to the mass analyzer, while neutral particles, which areunnecessary for the analysis and which may possibly contaminate thesystem and cause measurement noises, are removed before arriving at themass analyzer.

Effect of the Invention

In the ion guide according to the first or second aspect of the presentinvention, ions can be transported along a curved path in a more focusedform than in the conventional curved ion guides, so that a higher levelof ion transmission efficiency can be achieved. In the mass spectrometeraccording to the third aspect of the present invention which uses theion guide according to the first or second aspect of the presentinvention, the amount of ions to be subjected to the mass spectrometrywill be larger than in the case of using the conventional curved ionguide, so that the sensitivity and accuracy of the analysis willimprove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of an ion guide according toone embodiment (first embodiment) of the present invention.

FIG. 2 is a perspective view of the curved rod electrodes of the ionguide according to the first embodiment.

FIG. 3 is a schematic configuration diagram of a mass spectrometer usingthe ion guide according to the first embodiment.

FIG. 4 is a schematic configuration diagram of an ion guide according toanother embodiment (second embodiment) of the present invention.

FIGS. 5A and 5B are model diagrams comparing a direct-current electricfield in a conventional ion guide with a direct-current electric fieldin the ion guide according to the second embodiment.

FIG. 6 is a schematic configuration diagram of an ion guide according toanother embodiment (third embodiment) of the present invention.

FIGS. 7A and 7B are schematic configuration diagrams of an ion guideaccording to another embodiment (fourth embodiment) of the presentinvention.

FIG. 8 is a perspective view of the curved rod electrodes of a curvedion guide.

FIG. 9 is a diagram showing an electrode configuration and a circuitconfiguration of voltage sources in a conventional curved ion guide.

FIG. 10 is a diagram showing an electrode configuration and a circuitconfiguration of voltage sources in a conventional curved ion guide.

BEST MODE FOR CARRYING OUT THE INVENTION

The ion guide according to the present invention and the massspectrometer using the ion guide are hereinafter described by means ofembodiments.

First Embodiment

FIG. 1 is a schematic configuration diagram of an ion guide according tothe first embodiment, FIG. 2 is a perspective view of the curved rodelectrodes of the curved ion guide according to the first embodiment,and FIG. 3 is a schematic configuration diagram of a mass spectrometerhaving this curved ion guide.

As shown in FIG. 3, in the present mass spectrometer, ions generatedfrom a sample and ejected from an ionization unit (ion source) 1 areintroduced into a curved ion guide 2 for bending their path byapproximately 90°, in which the ions follow the curved central axis O ofthe ion guide 2, gradually bending their traveling direction, to beejected from the exit end of the ion guide 2. Neutral particles, such asthe sample molecules introduced from the ionization unit 1 into the ionguide 2 together with the ions, travel straightly, without beingaffected by the electric field within the ion guide 2, to be separatedfrom the ions and removed. The ions ejected from the exit end of the ionguide 2 are introduced into a mass analyzer 3, such as a quadrupole massfilter, in which the ions are separated according to theirmass-to-charge ratios and arrive at a detector 4.

As shown in FIG. 2, the ion guide 2 has four curved rod electrodes211-214 arranged around the curved central axis O. Among them, twocurved rod electrodes 212 and 214 have their centers located on a flatplane P (which corresponds to the plane of paper in FIG. 3) on which thecurved central axis O, which is a segment of an arc, lies. The other twocurved rod electrodes 211 and 213 have their centers located on a curvedsurface orthogonal to the flat plane P and including the curved centralaxis O. The curved rod electrodes 211-214 shown in FIG. 1 are their endfaces created by cutting the curved rod electrodes 211-214 at a planeorthogonal to the curved central axis O in FIG. 2.

As shown in FIG. 1, the voltage sources 522 and 523 apply aradio-frequency (RF) voltage V_(RF), with a predetermined direct-current(DC) bias voltage V_(BIAS) superposed thereon, to the two curved rodelectrodes 212 and 214 facing each other among the four curved rodelectrodes 211-214, as well as an RF voltage −V_(RF) which is identicalin amplitude and opposite in polarity to the RF voltage V_(RF), with thepredetermined DC bias voltage V_(BIAS) superposed thereon, to the othertwo curved rod electrodes 211 and 213. The DC bias voltage V_(BIAS) is avoltage applied to all the curved rod electrodes 211-214. This DC biasvoltage V_(BIAS) itself does not create any DC electric field within theion guide 2. It should be noted that FIGS. 9 and 10 are the examples ofDC bias voltage V_(BIAS)=0. As explained earlier, an RF electric fieldfor focusing ions while oscillating them is created within the ion guide2 by the RF voltages V_(RF) and −V_(RF) applied to the curved rodelectrodes 211-214. This is the same as in the conventional case.

The voltage source 522 applies, as the deflecting DC voltage, a DCvoltage −V_(DCx) whose polarity is opposite to that of an ion to beanalyzed (which is a positive ion in the present example), to the curvedrod electrode 212 located on the inside of the curved path. The factthat no deflecting DC voltage is applied to the curved rod electrode 214facing across the curved central axis O can be regarded as theapplication of a deflecting DC voltage of 0 V. By these voltages, a DCelectric field for attracting ions toward the inside of the curved path,i.e. in the direction indicated by the thick white arrow in FIG. 1, iscreated within the ion guide 2. The effect of this DC electric field isalso the same as in the conventional case.

Furthermore, in this ion guide 2, the voltage source 522 applies, as thefocusing DC voltage, a DC voltage −V_(DCy) having the same polarity asthat of the ion to be analyzed, to the two curved rod electrodes 211 and213 facing each other across the curved central axis O. The DC electricfield created in the vicinity of the curved rod electrodes 211 and 213by the application of this focusing DC voltage (the focusing DC electricfield) acts on the ions within the ion guide 2 so as to repel them fromthe curved rod electrodes 211 and 213. That is to say, as indicated bythe thick arrows in FIG. 1, the ions experience forces directed from theregions near the two curved rod electrodes 211 and 213 toward the curvedcentral axis O, so that they will not be easily spread outward, but willbe focused into the region near the curved central axis O, changingtheir traveling direction due to the effect of the deflecting DCelectric field. In the ion guide 2 of the present embodiment, the spreadof the ions is prevented by the combined effect of the focusing DCelectric field and the RF electric field, so that the ions can beefficiently transported along the curved central axis O to the exit end.

Second Embodiment

FIG. 4 is a schematic configuration diagram of a curved ion guideaccording to the second embodiment. In the second embodiment, thestructure and arrangement of the four rod electrodes 201-204 are not thesame as those of the first embodiment, but the same as those of theconventional examples shown in FIGS. 8-10. That is to say, none of thefour curved rod electrodes 201-214 are located on the flat plane P onwhich the curved central axis O lies or the curved surface orthogonal tothe flat plane P and including the curved central axis O. Similar to theconventional example shown in FIG. 10, a pair of deflecting auxiliaryelectrodes 205 and 206 facing each other across the curved central axisO is provided on the flat plane P on which the curved central axis Olies. Furthermore, a pair of focusing auxiliary electrodes 207 and 208facing each other across the curved central axis O is provided on thecurved surface orthogonal to the flat plane P and including the curvedcentral axis O. Each of the focusing auxiliary electrodes 207 and 208has a rectangular cross section and extends in a curved shape parallelto the curved central axis O.

As shown in FIG. 4, the voltage source 531 applies an RF voltage V_(RF),with a predetermined DC bias voltage V_(BIAS) superposed thereon, to thetwo curved rod electrodes 211 and 213 facing each other among the fourcurved rod electrodes 201-204. The voltage source 532 applies an RFvoltage −V_(RF) which is identical in amplitude and opposite in polarityto the RF voltage V_(RF), with the predetermined DC bias voltageV_(BIAS) superposed thereon, to the other two curved rod electrodes 212and 214. As a result, an RF electric field for focusing ions whileoscillating them is created within the ion guide 2.

The voltage source 533 applies, as the deflecting DC voltage, a DCvoltage V_(DCx) having the same polarity as that of the ion to beanalyzed, to the deflecting auxiliary electrode 206 located on theoutside of the curved path. The voltage source 534 applies, as thedeflecting DC voltage, a DC voltage −V_(DCx) whose polarity is oppositeto that of the ion to be analyzed, to the curved rod electrode 205located on the inside of the curved path. By these voltages, a DCelectric field for attracting ions toward the inside of the curved path,i.e. in the direction indicated by the thick white arrow in FIG. 4, iscreated within the ion guide 2.

Furthermore, in this ion guide 2, the voltage source 535 applies, as thefocusing DC voltage, a DC voltage −V_(DCy) having the same polarity asthat of the ion to be analyzed, to the focusing auxiliary electrodes 207and 208 facing each other across the curved central axis O. The DCelectric field created in the vicinity of the focusing auxiliaryelectrodes 207 and 208 by the application of this focusing DC voltageacts on the ions within the ion guide 2 so as to make them move awayfrom the curved rod electrodes 201-204.

FIGS. 5A and 5B are diagrams schematically showing equipotential linesdue to the DC electric field in a plane orthogonal to the curved centralaxis O, where FIG. 5A is a model diagram corresponding to a conventionalexample, and FIG. 5B is a model diagram corresponding to the secondembodiment shown in FIG. 3. As shown in FIG. 5A, in the conventionalsystem, the equipotential lines in the space surrounded by the curvedrod electrodes 201-204 are almost straight, in which ions will merelyexperience a force directed toward the inside of the curved central axisO. By contrast, as shown in FIG. 5B, in the system of the secondembodiment, the equipotential lines in the space surrounded by thecurved rod electrodes 201-204 are curved, with their middle portionsbulging leftward (or toward the outside of the curved central axis O).In this field, ions experience the resultant force of the force directedinward from the curved central axis O and the forces directed from thevicinity of the focusing auxiliary electrodes 207 and 208 toward thecurved central axis O. Accordingly, the ions will not be easily spreadoutward, but will be focused into the region near the curved centralaxis O, changing their travelling direction along the curvature of thecurved central axis O due to the effect of the deflecting DC electricfield. As a result, the ions will reach the exit end with highefficiency.

An RF voltage may additionally be superposed on the focusing DC voltageand applied to the focusing auxiliary electrodes 207 and 208 so as toassist the creation of the RF electric field.

Third Embodiment

FIG. 6 is a schematic configuration diagram of a curved ion guideaccording to the third embodiment. The ion guide according to the thirdembodiment has an octapole configuration with eight curved rodelectrodes 221-228. This system can be created by adding one curved rodelectrode between each and every pair of the curved rod electrodesneighboring each other in the circumferential direction in thequadrupole ion guide shown in the first embodiment. The voltage source541 and 544 apply an RF voltage V_(RF), with a predetermined DC biasvoltage V_(BIAS) superposed thereon, to the four curved rod electrodes221, 223, 225 and 227 which do not neighbor each other in thecircumferential direction (i.e. every other curved rod electrode). Thevoltage sources 542, 543 and 545 apply an RF voltage −V_(RF) which isidentical in amplitude and opposite in polarity to the RF voltageV_(RF), with the predetermined DC bias voltage V_(BIAS) superposedthereon, to the other four curved rod electrodes 222, 224, 226 and 228.An RF electric field for focusing ions while oscillating them is createdwithin the ion guide 2.

The voltage sources 541 and 542 apply, as the deflecting DC voltage, aDC voltage −V_(DEF) whose polarity is opposite to that of the ion to beanalyzed, to the three curved rod electrodes 221, 222 and 223 located onthe inside of the curved path. The voltage source 534 applies, as thedeflecting DC voltage, a DC voltage V_(DEF) having the same polarity asthat of the ion to be analyzed, to the three curved rod electrodes 225,226 and 227 located on the outside of the curved path. By thesevoltages, a DC electric field for attracting ions toward the inside ofthe curved path, i.e. in the direction indicated by the thick whitearrow in FIG. 6, is created within the ion guide 2. It is also possibleto apply the deflecting DC voltages to only the curved rod electrodes222 and 226.

Furthermore, the voltage source 543 applies, as the focusing DC voltage,a DC voltage −V_(DCy) having the same polarity as that of the ion to beanalyzed, to the two curved rod electrodes 224 and 228 facing each otheracross the curved central axis O. The DC electric field created in thevicinity of the curved rod electrodes 224 and 228 by the application ofthis focusing DC voltage acts on the ions within the ion guide 2 so asto push them from the curved rod electrodes 211 and 213 toward thecurved central axis O. Thus, similar to the previously describedembodiments, the ions will be curved along the curved central axis Owhile being prevented from spreading.

Fourth Embodiment

FIGS. 7A and 7B are schematic diagrams of a curved ion guide accordingto the fourth embodiment. Similar to the first embodiment, the ion guideaccording to the fourth embodiment has a quadrupole structure with noauxiliary electrodes. A difference exists in that curved virtual rodelectrodes are used in place of the curved rod electrodes. That is tosay, each curved virtual rod electrode is composed of a plurality ofplate electrodes (e.g. 231 a-231 f) arrayed at intervals along thecurved central axis O (the number of plate electrodes, which is six inthe example of FIG. 7B, may be any number), and there are four suchcurved virtual rod electrodes arranged at angular intervals of 90°around the curved central axis O. Although the plurality of plateelectrodes constituting one curved virtual rod electrode shown in FIG.7A are linearly arrayed, their positions should actually be shifted sothat they will be arrayed along the curvature of the curved central axisO. The reason for the straight appearance of the curved central axis Oin FIG. 7B is because FIG. 7B is a diagram showing end faces created bycutting the curved virtual rod electrodes in FIG. 7A at a curved surfaceorthogonal to the flat plane on which the curved central axis O lies andincluding the curved central axis O.

The voltage sources 553 and 554 apply an RF voltage V_(RF), with apredetermined DC bias voltage V_(BIAS) superposed thereon, to the plateelectrodes 232 a, 232 b, . . . , 234 a, 234 b, . . . included in the twocurved virtual rod electrodes facing each other across the curvedcentral axis O. The voltage source 551 applies an RF voltage −V_(RF)which is identical in amplitude and opposite in polarity to the RFvoltage V_(RF), with the predetermined DC bias voltage V_(BIAS)superposed thereon, to the plate electrodes 231 a, 231 b, . . . , 233 a,233 b, . . . included in the other two curved virtual rod electrodes. Asa result, an RF electric field for focusing ions while oscillating themis created within the ion guide 2.

The voltage source 553 applies, as the deflecting DC voltage, a DCvoltage −V_(DCx) whose polarity is opposite to that of the ion to beanalyzed, to the plate electrodes 232 a, 232 b, . . . included in thecurved virtual rod electrode located on the inside of the curved path.This is the same as the first embodiment, and by this voltage, a DCelectric field for attracting ions toward the inside of the curved path,i.e. in the direction indicated by the thick white arrow in FIG. 7A, iscreated within the ion guide 2.

Furthermore, the voltage source 551 applies, as the focusing DC voltage,a DC voltage V_(DCalt) having the same polarity as that of the ion to beanalyzed, to the foremost plate electrodes 231 a and 233 a as well asevery other subsequent plate electrode (231 c, 233 c, 231 e and 233 e)included in the two curved virtual rod electrodes facing each otheracross the curved central axis O. Similarly, the voltage source 552applies, as the focusing DC voltage, a DC voltage −V_(DCalt) whosepolarity is opposite to that of the ion to be analyzed, to the secondforemost plate electrodes 231 b and 233 b as well as every othersubsequent plate electrode (231 d, 233 d, 231 f and 233 f) included inthe two curved virtual rod electrodes facing each other across thecurved central axis O. The plate electrodes 231 a, 233 a, 231 c, 233 c,231 e and 233 e to which the DC voltage V_(DCalt) is applied function asconvex ion lenses for pushing ions toward the curved central axis O whenthe ions are passing through the spaces surrounded by these electrodes.On the other hand, the plate electrodes 231 b, 233 b, 231 d, 233 d, 231f and 233 f to which the DC voltage −V_(DCalt) is applied function asconcave ion lenses for pushing ions away from the curved central axis Owhen the ions are passing through the spaces surrounded by theseelectrodes. Thus, the ions are repeatedly focused and defocused as theymove forward, whereby the ions are efficiently transported to the exitend.

As described thus far, the ion guide according to any of the firstthrough fourth embodiments of the present invention transports ionswhile curving them along the curved central axis O and preventing thespread of the ions by the effect of the focusing DC electric field.Accordingly, as compared to conventional curved ion guides, it canachieve a higher level of ion transmission efficiency.

The ion guide according to the present invention can be used not only inthe section between the ionization unit and the mass analyzer, but alsoin various sections of the mass spectrometer in which it is necessary totransport ions to the subsequent stage while focusing them. For example,the previously described curved ion guide can be used as the ion guidecontained in a collision cell of a triple quadrupole mass spectrometer.Furthermore, the ion guide according to the present invention can beused not only in mass spectrometers but also in various kinds ofapparatuses or systems which require controlling the motion of ions.

It should be noted that any of the previously described embodiments is amere example, and any change, modification or addition appropriatelymade within the spirit of the present invention will evidently fallwithin the scope of claims of the present patent application. Forexample, although the ion guides in the previous embodiments are eithera quadrupole or octapole type, it is possible to adopt a hexapolestructure or a multi-pole structure with ten or more poles.

EXPLANATION OF NUMERALS

-   1 . . . Ionization Unit-   2 . . . Ion Guide-   2 . . . Curved Ion Guide-   201-204, 211-214, 221-228 . . . Curved Rod Electrode-   205, 206 . . . Deflecting Auxiliary Electrode-   207, 208 . . . Focusing Auxiliary Electrode-   231 a-231 f, 232 a-232 c, 233 a-233 f, 234 a-234 c . . . Plate    Electrode-   3 . . . Mass Analyzer-   4 . . . Detector-   521-523, 531-535, 541-545, 551-554 . . . Voltage Source-   O . . . Curved Central Axis

1. An ion guide for transporting ions along a curved path while focusingthe ions, comprising: a) 2n pieces of curved rod electrodes (n is aninteger equal to or greater than two) arranged around a curved centralaxis; and b) a voltage generator for applying voltages to the 2n piecesof curved rod electrodes as follows: radio-frequency voltages withopposite polarities are applied to any two curved rod electrodesneighboring each other in a circumferential direction among the 2npieces of curved rod electrodes; a deflecting direct-current voltage isapplied to at least one of the curved rod electrodes in addition to theradio-frequency voltages, so as to attract ions in a space surrounded bythe 2n pieces of curved rod electrodes toward an inside of a curvatureof the curved central axis in a plane orthogonal to the curved centralaxis; and a focusing direct-current voltage is applied to at least twocurved rod electrodes facing each other across the curved central axis,exclusive of the curved rod electrodes to which the deflectingdirect-current voltage is applied, in addition to the radio-frequencyvoltages, so as to push the ions in the space surrounded by the 2npieces of curved rod electrodes from both sides toward the curvedcentral axis, in the plane orthogonal to the curved central axis andalong a line orthogonal or oblique to a direction in which the ions areattracted due to the deflecting direct-current voltage.
 2. The ion guideaccording to claim 1, wherein: the ion guide has a quadrupole structureof n=2 with four curved rod electrodes arranged in such a manner thatone pair of the curved rod electrodes facing each other across thecurved central axis have centers thereof located on a flat plane onwhich the curved central axis lies while another pair of the curved rodelectrodes have centers thereof located on a curved surface orthogonalto the flat plane and including the curved central axis; and the voltagegenerator applies the deflecting direct-current voltage to one or bothof the pair of the curved rod electrodes having the center thereoflocated on the flat plane and the focusing direct-current voltage havinga same polarity as that of an ion to be analyzed to the other pair ofthe curved rod electrodes.
 3. The ion guide according to claim 1,wherein: each of the curved rod electrodes is a curved virtual rodelectrode composed of an array of plate electrodes arranged along thecurved central axis; and the voltage generator applies, as the focusingdirect-current voltage, a voltage having a same polarity as that of anion to be analyzed and a voltage having an opposite polarity, to thearray of the plate electrodes constituting one curved virtual rodelectrode so that these two voltages alternate in the array.
 4. An ionguide for transporting ions along a curved path while focusing the ions,comprising: a) 2n pieces of curved rod electrodes (n is an integer equalto or greater than two) arranged around a curved central axis, with noneof the curved rod electrodes being located on a flat plane on which thecurved central axis lies; b) a deflecting auxiliary electrode having acurved shape, located on the flat plane on which the curved central axislies and between the curved rod electrodes neighboring each other in acircumferential direction; c) a focusing auxiliary electrode having acurved shape, located on a curved surface which is orthogonal or obliqueto the flat plane and which includes the curved central axis and betweenthe curved rod electrodes neighboring each other in the circumferentialdirection; d) a main voltage generator for applying radio-frequencyvoltages with opposite polarities to any two curved rod electrodesneighboring each other in the circumferential direction among the 2npieces of curved rod electrodes; and e) an auxiliary voltage generatorfor applying a deflecting direct-current voltage to the deflectingauxiliary electrode so as to attract ions in a space surrounded by the2n pieces of curved rod electrodes toward an inside of a curvature ofthe curved central axis in a plane orthogonal to the curved centralaxis, and for applying a focusing direct-current voltage to the focusingauxiliary electrode so as to push the ions in the space surrounded bythe 2n pieces of curved rod electrodes from both sides toward the curvedcentral axis, in the plane orthogonal to the curved central axis andalong a line orthogonal or oblique to a direction in which the ions areattracted due to the deflecting direct-current voltage.
 5. The ion guideaccording to claim 4, wherein: the ion guide has a quadrupole structureof n=2 with one pair of the deflecting auxiliary electrodes facing eachother across the curved central axis and one pair of the focusingauxiliary electrodes facing each other across the curved central axis ona curved surface orthogonal to the flat surface; and the auxiliaryvoltage generator applies a deflecting direct-current voltage whosepolarity is opposite to that of an ion to be analyzed to one of thedeflecting auxiliary electrodes located on the inside of the curvature,a deflecting direct-current voltage having a same polarity as that ofthe ion to be analyzed to another one of the deflecting auxiliaryelectrodes located on the outside of the curvature, and a focusingdirect-current voltage having the same polarity as that of the ion to beanalyzed to both of the focusing auxiliary electrodes.
 6. A massspectrometer having an ion guide provided between an ion source and amass analyzer, the ion guide comprising: a) 2n pieces of curved rodelectrodes (n is an integer equal to or greater than two) arrangedaround a curved central axis; and b) a voltage generator for applyingvoltages to the 2n pieces of curved rod electrodes as follows:radio-frequency voltages with opposite polarities are applied to any twocurved rod electrodes neighboring each other in a circumferentialdirection among the 2n pieces of curved rod electrodes; a deflectingdirect-current voltage is applied to at least one of the curved rodelectrodes in addition to the radio-frequency voltages, so as to attractions in a space surrounded by the 2n pieces of curved rod electrodestoward an inside of a curvature of the curved central axis in a planeorthogonal to the curved central axis; and a focusing direct-currentvoltage is applied to at least two curved rod electrodes facing eachother across the curved central axis, exclusive of the curved rodelectrodes to which the deflecting direct-current voltage is applied, inaddition to the radio-frequency voltages, so as to push the ions in thespace surrounded by the 2n pieces of curved rod electrodes from bothsides toward the curved central axis, in the plane orthogonal to thecurved central axis and along a line orthogonal or oblique to adirection in which the ions are attracted due to the deflectingdirect-current voltage.
 7. The mass spectrometer according to claim 6,wherein: the ion guide has a quadrupole structure of n=2 with fourcurved rod electrodes arranged in such a manner that one pair of thecurved rod electrodes facing each other across the curved central axishave centers thereof located on a flat plane on which the curved centralaxis lies while another pair of the curved rod electrodes have centersthereof located on a curved surface orthogonal to the flat plane andincluding the curved central axis; and the voltage generator applies thedeflecting direct-current voltage to one or both of the pair of thecurved rod electrodes having the center thereof located on the flatplane and the focusing direct-current voltage having a same polarity asthat of an ion to be analyzed to the other pair of the curved rodelectrodes.
 8. The mass spectrometer according to claim 6, wherein: eachof the curved rod electrodes is a curved virtual rod electrode composedof an array of plate electrodes arranged along the curved central axis;and the voltage generator applies, as the focusing direct-currentvoltage, a voltage having a same polarity as that of an ion to beanalyzed and a voltage having an opposite polarity, to the array of theplate electrodes constituting one curved virtual rod electrode so thatthese two voltages alternate in the array.
 9. A mass spectrometer havingan ion guide provided between an ion source and a mass analyzer, the ionguide comprising: a) 2n pieces of curved rod electrodes (n is an integerequal to or greater than two) arranged around a curved central axis,with none of the curved rod electrodes being located on a flat plane onwhich the curved central axis lies; b) a deflecting auxiliary electrodehaving a curved shape, located on the flat plane on which the curvedcentral axis lies and between the curved rod electrodes neighboring eachother in a circumferential direction; c) a focusing auxiliary electrodehaving a curved shape, located on a curved surface which is orthogonalor oblique to the flat plane and which includes the curved central axisand between the curved rod electrodes neighboring each other in thecircumferential direction; d) a main voltage generator for applyingradio-frequency voltages with opposite polarities to any two curved rodelectrodes neighboring each other in the circumferential direction amongthe 2n pieces of curved rod electrodes; and e) an auxiliary voltagegenerator for applying a deflecting direct-current voltage to thedeflecting auxiliary electrode so as to attract ions in a spacesurrounded by the 2n pieces of curved rod electrodes toward an inside ofa curvature of the curved central axis in a plane orthogonal to thecurved central axis, and for applying a focusing direct-current voltageto the focusing auxiliary electrode so as to push the ions in the spacesurrounded by the 2n pieces of curved rod electrodes from both sidestoward the curved central axis, in the plane orthogonal to the curvedcentral axis and along a line orthogonal or oblique to a direction inwhich the ions are attracted due to the deflecting direct-currentvoltage.
 10. The mass spectrometer according to claim 9, wherein: theion guide has a quadrupole structure of n=2 with one pair of thedeflecting auxiliary electrodes facing each other across the curvedcentral axis and one pair of the focusing auxiliary electrodes facingeach other across the curved central axis on a curved surface orthogonalto the flat surface; and the auxiliary voltage generator applies adeflecting direct-current voltage whose polarity is opposite to that ofan ion to be analyzed to one of the deflecting auxiliary electrodeslocated on the inside of the curvature, a deflecting direct-currentvoltage having a same polarity as that of the ion to be analyzed toanother one of the deflecting auxiliary electrodes located on theoutside of the curvature, and a focusing direct-current voltage havingthe same polarity as that of the ion to be analyzed to both of thefocusing auxiliary electrodes.